A process for measuring the thermal power released locally in an area of the core of a nuclear reactor is already known (French Application No. FR-A-2,420,827). This process measures the voltage at the terminals of a measuring thermocouple with a first junction brought to a reference temperature directly connected to the temperature of the coolant of the reactor at said area during stabilized operation, and a second junction placed so as to be brought to the temperature of a body which absorbs the gamma radiation at said area.
Thus, a measurement is performed by a differential thermocouple subjected to temperature gradients which exist between a mass which absorbs the gamma radiation and is therefore brought to a temperature which depends on the gamma flux and on the local power which is directly connected to it, and a heat source at a temperature which is constant in permanent stabilized operation.
However, during the normal or accidental transitory changes of the operation of the reactor, the temperature of the reference junction develops much more rapidly than the temperature of the hot junction. As a consequence, the output signal obtained comes from the superpositioning of a component representing the gamma flux absorbed and of a transitory component originating in the temperature variations of the reactor coolant.